A condenser microphone includes a microphone unit in which a diaphragm and a backplate are arranged so as to be opposed to each other. The microphone unit incorporates an impedance converter because of its very high impedance. As the impedance converter, a field effect transistor (FET) is usually used, and on rare occasions, a vacuum tube is used.
An electronic circuit for audio output of the condenser microphone is housed in a metallic cylindrical microphone case in a state of being mounted on a substrate. Usually, at one end of the microphone case, a connector housing section is provided, and in the connector housing section, a three-pin type output connector (three-pin output connector) specified in EIAJ RC5236 (Audio latch lock round type connector) is mounted.
When the microphone is used, the output connector is connected to a phantom power source via an output cable (balanced shielded cable) having a connection plug that is the mate to the output connector. The connection plug is provided with three female contacts mating with three pins (grounding, signal hot side, and signal cold side) of the output connector, and also a metallic outer ring connected to a shield coated line is provided. When the output connector is inserted in the connector housing section, the outer ring comes into contact with the inner surface of the connector housing section, and is electrically connected to the microphone case.
If strong electromagnetic waves radiated from a cellular phone or the like are applied to the microphone or the output cable, the electromagnetic waves pass through the output cable and intrude into the microphone via the output connector. In the microphone, the electromagnetic waves are sometimes demodulated by the impedance converter and delivered from the microphone as noise having an audio frequency.
To prevent this phenomenon, No. 1 pin for grounding of the three pins that the output connector has is connected to the microphone case, and the outer ring of the connection plug connected to the shield coated line is brought into contact with the inner surface of the microphone case (inner surface of the connector housing section) to provide electrical connection, by which a shielding function is given to the microphone case.
However, since the connector housing section and the outer ring of connection plug engage with each other in a cylindrical male-and-female engagement manner, the contact is a point contact, and this point contact portion has high frequency impedance, which provides incomplete shield. Also, if looseness due to a dimensional error etc. is present between the connector housing section and the outer ring of connection plug, the contact point is unfixed, and the shield becomes unstable. Such incompleteness and instability of shield bring about the generation of noise due to electromagnetic waves.